use codecov

* Avoid using KEYS.

* Check single keys, or patterns with `SCAN`

Continues the work that @Tinche did in #149.

* Add an arg, `check-single-keys` to get size and value with `TYPE`
* Modify the behavior of `check-keys`
  * Get keys from globs with `SCAN` rather than `KEYS`
  * If a key doesn't have any glob characters in it, just add it to the
    list of things to check with `TYPE`

I added an argument rather than decide `SCAN` vs `TYPE` based on the
presence of glob characters because, technically, redis key names can
contain glob characters, according to https://redis.io/topics/data-types-intro:

    Redis keys are binary safe, this means that you can use any binary
    sequence as a key, from a string like "foo" to the content of a JPEG
    file. The empty string is also a valid key.

The only way to be able to check for key names with glob characters in
them is by being able to explicitly specify keys that you don't want to
glob for.

* add empty help text to be compatible with prometheus >= 2.3.1

* Properly decode uptime from redis
* Add uptime_in_seconds & start_time_seconds to expected metrics

Add alternative credentials claim for host.

Add start_time_seconds

Adresses #167

start_time is a more idiomatic way to store the process uptime,
and is what is provided by the Prometheus library itself.

Since this is a breaking change, I am happy to add this as a second
metric if necessary

add slow logs count

Optional Redis Lua script for metric collection

Allow an optional Redis Lua script to collect extra metrics.

This can be enabled with a new `-script` flag. An example is provided in
contrib.

remove spurious v from /metrics

fix build instructions

Add `maxclients` to config metrics

sample k8s deployment yaml for redis with redis_exporter as a sidecar